Canopy cover

The report "Green carbon, the role of natural forests in carbon storage" says a difference in the definition of a forest was also underestimating the carbon stock in old-growth forests.

The IPCC defines a forest as trees taller than two metres and a canopy cover greater than 10%, but in Australia a forest is defined as having trees taller than 10 metres and a canopy cover greater than 30%.

The report says southeast Australia's unlogged forests could store about 640 tonnes per hectare, yet the IPCC estimate puts it at only around 217 tonnes of carbon per hectare.

The scientists estimate that around 9.3 billion tonnes of carbon can be stored in the 14.5 million hectares of eucalypt forests in southeast Australia if they are left undisturbed.

In Australia the largest stocks of carbon are found in the tall wet eucalypt forests of Victoria and Tasmania.

These forests support trees up to 80 metres tall and can contain more than 1200 tonnes of carbon per hectare, which is up to 10 times more carbon per hectare than previously realised.

The IPCC estimates only one-third of this capacity and only 27% of the forests' biomass carbon stock.

'More resilient'

Not only did natural forests store more carbon, but because they remained untouched they stored the carbon for longer than plantation forests which were cut down on a rotation basis.

The report says "natural forests are more resilient to climate change and disturbances than plantations".

Co-author of the report Professor Brendan Mackey, of ANU's Fenner School of Environment and Society, says protecting natural forests serves two purposes: it maintains a large carbon sink and stops the release of the forest's stored carbon.

"Protecting the carbon in natural forests is preventing an additional emission of carbon from what we get from burning fossil fuel," Mackey says.

The carbon stored in the world's biomass and soil is about three times the amount in the atmosphere, according to the report.

About 35% of greenhouse gases in the atmosphere is a result of past deforestation and 18% of annual global emissions is from continued deforestation.

Impact of logging

The report says logging results in more than a 40% reduction in long-term carbon compared with unlogged forests.

"The majority of biomass carbon in natural forests resides in the woody biomass of large old trees. Commercial logging changes the age structure of forests so that the average age of trees is much younger," it says.

"The carbon stock of forests subject to commercial logging, and of monoculture plantations in particular, will therefore always be significantly less on average than the carbon stock of natural, undisturbed forests."

The scientists say stopping further deforestation of southeast Australia's eucalypt forests is the equivalent of preventing emissions of 460 million tonnes of carbon dioxide a year for the next 100 years.

Allowing logged forests to regrow to their natural carbon storage capacity will avoid emissions of 136 million tonnes of carbon dioxide a year for the next 100 years, about 25% of Australia's total emissions in 2005.

"In Australia and probably globally the carbon carrying capacity of natural forests is underestimated and therefore misrepresented in economic valuations and in policy options," the report says.